This invention relates to a marine propulsion system, and more particularly to an exhaust discharge system for a marine propulsion system.
In a marine propulsion system, it is known to route exhaust gas through an internal exhaust discharge passage formed in the housing of the marine propulsion system. In an outboard motor, for example, the exhaust discharge system typically includes an exhaust passage provided in the lower unit of the motor which houses the driveshaft and the gearcase. A propeller is generally mounted to a propeller shaft disposed in the lower end of the lower unit, and it is desirable to provide an exhaust passage which discharges exhaust through the hub of the propeller. Such discharge of the exhaust provides quiet operation of the motor and enhances performance. A similar manner of discharging exhaust is used in connection with a stern drive system.
It is also desirable to minimize the surface area of that portion of the lower unit which is disposed below the water level during normal operation of a boat, in order to reduce drag developed by such submerged portion. This desirability for reducing the surface area of the submerged portion of the lower unit housing results in a configuration of the lower portion of the gearcase which provides a restricted area for exhaust flow through the exhaust discharge passage formed therein. This restricted area of exhaust flow is normally located immediately upstream of the entrance of the exhaust into the propeller shaft housing. At certain operating conditions, such area of restricted exhaust flow can cause a bottleneck in the exhaust discharge system which is unable to accommodate the exhaust output produced by the engine. In this circumstance, the performance of the engine suffers.
The present invention is designed to overcome the above-noted shortcomings in the exhaust discharge system for a marine propulsion system. In accordance with the invention, in an exhaust system including an area of restricted exhaust flow, relief means is provided adjacent the area of restricted exhaust flow for relieving exhaust pressure. In one embodiment, the relief means comprises one or more openings in the housing of the lower gearcase for providing an alternate outlet for exhaust adjacent the area of restricted flow. Closure means is provided adjacent the one or more openings for selectively opening and closing the openings for regulating the flow of exhaust gas therethrough. The closure means may be responsive to the pressure of exhaust within the exhaust passage adjacent the area of restricted exhaust flow. In one embodiment, the closure means comprises one or more plates adapted to cover the one or more openings, with the plates being movable between an open position and a closed position. A bias means acts to bias the one or more plates toward their closed position, and the bias means is designed so as to yield to a predetermined exhaust pressure within the exhaust passage so as to allow the one or more plates to move to their open position. In one embodiment, the one or more plates are formed of a resilient material, such as a spring steel, and are mounted to the gearcase housing so that the resilience of the material biases the one or more plates toward their closed position.